There is a great deal of interest in developing better and more efficient methods for storing energy for applications such as radio communication, satellites, portable computers, and electric vehicles, to name but a few. Accordingly, there have been recent concerted efforts to develop high energy, cost-effective battery cells having improved performance characteristics. Electrochemical battery cells are preferred and hence widely used in these applications since the chemical reactions that take place in the cells can be converted into useful electrical energy. An electrochemical battery cell uses its reactive components, namely the positive and negative electrodes, to generate an electric current. The electrodes are separated from one another by an electrolyte which maintains the simultaneous flow of ionic conduction between the two electrodes. Electrons flow from one electrode through an external circuit to the other electrode completing the circuit.
Rechargeable, or secondary, cells are more desirable than primary (non-rechargeable) cells since the associated chemical reactions are reversible. Accordingly electrodes for secondary cells must be capable of being regenerated (i.e., recharged) many times. The development of advanced rechargeable battery cells depends on design and selection of appropriate materials for the electrodes and the electrolyte.
Present day cells use alkali metals and ions as electroactive species. These cells obtain maximum voltage when the negative electrode is the zero-valent metal. Alloys of lithium and/or other low voltage intercalation materials, such as carbon, are sometimes used as the active component in the negative electrode. State-of-the-art positive electrodes can be fabricated from a material having the empirical formula A.sub.x M.sub.y O.sub.z, where A is a metal selected from groups IA or IIA of the periodic chart, M is a transition metal, O is oxygen and .sub.x,y, and .sub.z represent the relative combining ratios of each element. Positive electrodes manufactured with materials having this formula are fully disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 08/116,646 filed Sep. 7, 1993, the disclosure of which is incorporated herein by reference. Other types of positive electrode materials commonly known in the art may also be employed.
Currently, electrolytes disposed between the positive and negative electrodes include a polymer, such as poly-ethylene-oxide (PEO), impregnated with a salt in a solvent. Preferred salts include, for example, LiPF6. Ethylene carbonate, diethyl carbonate and combinations thereof are used as the solvent. A schematic representation of the prior art cell is illustrated in FIG. 1. Specifically, FIG. 1 illustrates a positive electrode 10, and a negative electrode 20 as described hereinabove. Disposed between said positive and negative electrodes is the electrolyte layer 30. Positive electrode/electrolyte interface 32 is created at the boundary between the positive electrode 10 and the electrolyte 30. Similarly, a negative electrode/electrolyte interface 34 is created at the boundary of the negative electrode 20 and the electrolyte 30. As may be appreciated from a perusal of FIG. 1, the electrolyte is in direct contact with both the negative and positive electrodes.
Present day, state-of-the-art polymer electrolyte batteries are often plagued however by electrode/electrolyte incompatibilities. For example, at the negative electrode/electrolyte interface 34, the negative electrode and the electrolyte solvent may react to form an ionically insulating layer on the surface of the negative electrode material. This ionically insulating layer blocks active material reaction sites, effectively isolating the negative electrode. The result is reduced capacity with cycling, (i.e., repeated cell charge/discharge) and poor cycle life.
At the positive electrode/electrolyte interface 32, problems include thermodynamic instability of the electrolyte due to the higher electrode potentials of the positive electrode. This causes electrolyte decomposition, and electrolyte out-gassing. The result of outgassing and decomposition is increased internal cell pressure, reduced cell performance, and ultimately, explosive cell failure. Until now, the problem of electrolyte decomposition has been addressed by operating the cell at less than the positive electrode's half cell potential. That is at a potential at which the electrolyte remains stable. This, however, results in decreased energy density, and decreased charge per cycle.
Accordingly, there exists a need for an electrolyte which has enhanced conductivity, yet reduces the occurrence of deleterious reactions taking place at the negative and positive electrode/electrolyte interfaces.